Method and apparatus for non-destructive inspection of tires

ABSTRACT

A pulsed through-transmission ultrasonic non-destructive inspection of the internal structure in a tire wall is effected. The transmitted acoustic signals preferably have a frequency of at least 40 KHz, are transduced to electrical form, amplified with a relatively long time constant AGC, rectified and integrated during only the initial or leading edge portions of each pulse or burst. The resulting integrated analog signal values thus provided have relative magnitudes which may be displayed or otherwise processed to detect structural anomalies within the tire wall. If plural acoustic transmitters are utilized, they are preferably multiplexed such that only a single transducer is activated at a given time. Air leaks from the pressurized tire under test are also detected by detecting ultrasonic signals generated by escaping air.

This application is a division of my earlier copending application Ser.No. 31,962 filed Apr. 19, 1979 now U.S. Pat. No. 4,285,235 dated Aug.25, 1981.

This invention is generally directed to methods and apparatus fornon-destructive inspection or rubber tires. Such inspection techniquesmay also be combined with conventional tire buffing operations inaccordance with this invention.

The invention here claimed is directed to certain air leak detectionfeatures of the preferred embodiment. The mechanical features, per seare the sole invention of Doyle L. Dugger and are claimed in commonlyassigned copending application No. 31,961 filed Apr. 19, 1979 now U.S.Pat. No. 4,266,428 dated May 12, 1981 and in a divisional applicationSer. No. 233,967 filed Feb. 12, 1981. The combination of mechanical andelectrical features, is the joint invention of myself and Doyle L.Dugger and is claimed in commonly assigned copending application No.31,963 filed Apr. 19, 1979 now U.S. Pat. No. 4,275,589 dated June 30,1981.

There has long been an urgent need for cost effective, efficient,non-destructive inspection (NDI) of rubber tire casings. There areobvious safety benefits to be had by such techniques if they can beefficiently and rapidly practiced. There are also potential economicbenefits. For example, during tire retreading operations, a defectivetire carcass can be discarded before wasting further expenditures oftime and money if it can be accurately, efficiently and quicklydetected.

In fact, the need for improved NDI methods and apparatus relating to thetesting of tire casings is so great that the Army Materials andMechanics Research Center has sponsored special symposia devotedentirely to this subject in 1973, 1974, 1976 and 1978. The proceedingsof the first three of these symposia have now been published and areavailable from the National Technical Information Service. They eachinclude a complete chapter on ultrasonic tire testing as well as otherchapters devoted to different tire testing procedures (e.g. holographic,infrared and X-ray). There are also many prior art patents relatinggenerally to the use of ultrasonic waves to non-destructively testpneumatic tire casings. For example: U.S. Pat. Nos. 2,345,679--Linse(1944), 2,378,237--Morris (1945), 3,336,794--Wysoczanski et al (1967),3,604,249--Wilson (1971), 3,815,407--Lavery (1974), 3,882,717--McCauley(1975), 4,059,989--Halsey (1977).

There are also several prior art patents relating to mechanicalstructures for chucking or otherwise physically handling pneumatic tirecasings during various types of non-destructive testing or manufacturingprocesses. For example: U.S. Pat. Nos. 2,695,520--Karsai (1954),3,550,443--Sherkin (1970), 3,948,094--Honlinter (1976),4,023,407--Vanderzee (1977).

Although a wide variety of non-destructive ultrasonic tests have beenperformed on tires in the past are shown by these prior art patents,they have each suffered serious deficiencies and have failed to achievewidespread acceptance in commercial practice. Some of the prior artapproaches have required a liquid coupling medium on one or both sidesof the tire wall under test. Some prior testing procedures use aso-called "pulse-echo" approach which gives rise to a rather complexpattern of echos due to normal internal tire structures as well as forabnormal structures. Many have used relatively low frequencies (e.g. 25Khz) with resulting severe interference from normal ambient acousticsources while others have used extremely high frequencies (e.g. 2 Mhz)with resulting rapid signal attenuation. Some have used continuousultrasonic waves resulting in a confusing pattern of standing waves andthe like while others have looked for envelope peaks in the receivedacoustic waves. Others have used individual pulses of acoustic signalsfor each tire testing site. In some cases the peak received envelopemagnitude has been used to reach final data values. Some have alsoattempted to test an inflated tire carcass (but sometimes causing theacoustic signals to pass through two tire walls so as to keep alltransducers external to the tire) although most have attempted to test anoninflated tire carcass. There may have been other techniques as well.

It has now been discovered that these earlier attempts at ultrasonicnon-destructive inspection of tire casings can be considerably improvedand made more commercially viable.

For example, it has been discovered that a pulse or burst transmissionmode may be used to reduce standing waves or unwanted reverberationeffects within the tire. Each burst comprises only a few (e.g. 100)cycles of acoustic signals providing a very low overall duty cycle andextremely efficient transducer operation. At the same time, it has beendiscovered that the envelope of received acoustic signals may be alteredby internal reverberation, standing wave, wave cancellation or otherirrelevant wave effects after the initial portion or rising edge of eachburst is received. Accordingly, in the preferred embodiment of thisinvention, the received acoustic signals are passed through a gatedreceiver circuit such that only those signals within the initial portionof each burst are utilized.

Still further improvements may be possible in some circumstances byaveraging readings taken at different frequencies thereby avoiding somepossible adverse standing wave pattern effects and the like.Furthermore, non-linear analog-to-digital conversion techniques may beused to assist in recovering usable data.

In the presently preferred embodiment, plural transmitting acoustictransducers are located inside a revolving inflated tire so as toacoustically illuminate the entire inside tire surfaces under test.However, it has been discovered that peculiar wave cancellation,standing wave patterns or similar wave effects may distort readings ifmore than one transmitter is activated at a given time. Accordingly, thepreferred embodiment includes multiplexing circuitry to insure that onlya single transducer is activated at a given time.

Plural acoustic receiving transducers are arrayed about the outer tirewalls so as to receive acoustic signals transmitted therethrough fromthe transmitting transducers located inside the tire. Each receivingtransducer is preferably collimated and matched to the ambient airimpedance with a cylindrical tube having an inner conical surface whichtapers down to the sensing area of the actual receiving transducer. Suchcollimation helps to confine each receiver's output to representacoustic signals transmitted through a limited area of the tire wall andfurther helps to reject interference from tread patterns and ambientnoise. Flaws in the tires such as separations between cord layers andrubber layers or between various rubber layers attenuate the acousticsignals passing therethrough to a greater extent than when the acousticsignals pass through a normal section of the tire wall.

It has also been discovered that leaks in in a pressurized tire (i.e.air passing through the tire wall) can be detected with the sameultrasonic receiving transducers by noting an increase in receivedsignal level over that encountered during passage by normal sections ofthe tire wall even while the ultrasonic transmitters are turned off.

Each of the receiving transducers is connected to its own signalprocessing channel albeit plural receivers may be multiplexed to share acommon signal processing channel in synchronization with themultiplexing of the plural acoustic transmitters thereby minimizing thenumber of necessary signal processing channels. A relatively long termautomatic gain controlled amplifier is incorporated in each signalprocessing channel so as to compensate for different average signallevels from tire-to-tire and from channel-to-channel, depending upondifferent average respective tire wall thicknesses. After AGCamplification, the received ultrasonic signals are rectified andintegrated during a gated period on the rising edge of each burst. Theresulting integrated values then truly represent the relativetransmission capabilities of different successive sections of the tirewall under inspection. In one exemplary embodiment, successiveobservations at each tire wall position are averaged together to avoidpotential standing wave null points and the like which might occur atsome receiver locations for some particular frequency and tire geometry.Such values may be displayed on a CRT for visual inspection anddetection of defects. Alternatively, such values may be digitized(possibly with a nonlinear exponential-law A-to-D process to enhance theeffective signal-to-noise ratio at relatively low signal strengths)before display and/or process desired pattern recognition algorithms ina digital computer so as to automatically identify tire anomalies suchas separations between layers.

It has also been discovered that improved operation results when theacoustic signals are of a moderately high frequency (e.g. greater thanapproximately 40 Khz and, in the preferred embodiment, 75 Khz). Suchmoderately high acoustic frequencies tend to avoid unwanted spuriousindications caused by the usual ambient acoustic sources and, at thesame time, provide relatively short wave lengths (e.g. approximately 1.5inches or so in tire rubber) thereby improving the resolution ofrelatively small tire defects, yet without unnecessarily complicatingthe observed transmission readings by having a wave length so small thatthe signals may be affected by tire structure anomalies presenting noactual defect.

The averaging of received signal over several cycles during the leadingedge of each burst improves the signal-to-noise ratio of the resultingmeasured values as does the use of a non-linear A to D process. Theaveraging of data taken at different frequencies may further enhance theresults.

The use of an inflated tire in the preferred embodiment has beendiscovered to assist in maintaining a true running tire surface and thusavoids signal variations that might otherwise be caused by wobbling orother relative axial motions of the tire walls during rotation. Theinflated tire is also useful in helping to at least partially stress thetire walls, as they will be stressed during normal use, and to open upleakage passageways through the tire walls so that they may be detectedby ultrasonic detection of air passing therethrough. Approximately onlyfive psi is needed to maintain a stable inflated tire structure.However, it has been discovered that improved signal transmission andoverall performance occurs if the tire is inflated within the range ofapproximately 15-18 psi.

Although it may not be required, it is preferred that the outertreadwall of the tire under inspection first be buffed to present auniform surface thus minimizing spurious defect indications that mightotherwise be caused by tread patterns and/or by uneven wear spots orpatterns in the outer treadwall surface of the tire. In this connection,the tire buffing apparatus and method may be advantageously employed incombination with the ultrasonic non-destructive testing method andapparatus to present a unified, convenient and efficient overalloperation. Since such a buffing operation is necessarily involved intire retreading operations anyway, this combination is particularlyattractive where the tire carcasses are being inspected in preparationfor retreading.

The ultrasonic bursts and receiver gating periods are preferablysynchronized to occur at corresponding successive incremental positionsof the rotating tire such that the final display or defect indicationmay be accurately located with respect to an index mark on the tireand/or tire mounting flange or the like.

These and other objects and advantages of this invention will be betterappreciated by reading the following detailed description of thepresently preferred exemplary embodiment in conjunction with theaccompanying drawings, of which:

FIGS. 1 and 2 are perspective views of a combined NDI/buffer machineconstructed in accordance with this invention;

FIG. 3 is a block diagram of the invention shown in FIGS. 1 and 2;

FIG. 4 is a block diagram of the ultrasonic NDI circuits which may beused in the NDI/buffer machines of FIGS. 1-3 or in a machine having onlyNDI capabilities;

FIG. 5 includes a schematic showing of a tire wall section, acoustictransmitters and receivers and of the pre-amplifier and multiplexingcircuitry shown in FIG. 4;

FIG. 6 is a detailed circuit diagram of the pre-amplifier shown in FIG.5;

FIG. 7 is a detailed circuit diagram of a representative one of thesignal processing channels shown in FIG. 4;

FIGS 8a and 8b comprise a detailed circuit diagram of the systeminterface shown in FIG. 4;

FIG. 9 is a detailed circuit diagram of the CPU or central processingunit shown in FIG. 4;

FIG. 10 is a detailed circuit diagram of the display interface shown inFIG. 4;

FIG. 11 is a schematic depiction of several representative wave formsuseful in explaining the operation of the circuits shown in FIGS. 4-10;

FIG. 12 is a cross-sectional view of a collimater/impedance matchingdevice used in each of the receiving transducers;

FIGS. 13 and 14 are tracings of CRT outputs obtained bynon-destructively inspecting a buffed tire carcass in accordance withthis invention;

FIGS. 15 and 16 are flow diagrams of a suitable control program for usewith the CPU of FIGS. 4-10;

FIG. 17 is a diagram of another circuit for generating the AGC amplifierand integrator channels; and

FIGS. 18, 19a and 19b illustrate a program sequence which searches forair leaks and then searches for separations in two eight-channel groups.

Referring to FIGS. 1 and 2, two perspective view of the presentlypreferred exemplary combined tire buffer and NDI machine are shown. Aswill be apparent, the NDI features of the machine may be provided, ifdesired, without including the tire buffing capability.

The major mechanical components of the machine are mounted to an openframe 100 having a fixed spindle 102 and an axially movable spindle 104opposingly aligned along horizontal axis 106. Conventional circular tirecounting rings or flanges 108 and 110 are attached to the outerrotatable ends of spindles 102 and 104 for mounting an inflated tire112, therebetween. A conventional pneumatically operated tire liftmechanism 114 is conveniently provided so as to assist the humanoperator in lifting and swinging a tire into and out of place betweenrings 108 and 110 during tire mounting and demounting operations.

Ring 108, and hence tire 112, is driven by a two horsepower d.c. motor116 through reducing gears 118. A tire surface speed of approximately600 feet per minute is preferred for buffing operations while a muchlower speed of approximately 40 feet per minute is preferred for NDIoperations. Spindle 104, and hence ring 110, is axially extended andretracted by pneumatic cylinder 120. During tire mounting operations,ring 110 is retracted by cylinder 120 so as to permit the tire 112 to belifted into place on ring 108 by lift 114. Thereafter, ring 110 isextended against the corresponding rim of tire 112 and the tire isinflated to a desired set point pressure by compressed air passedthrough the center of spindle 102.

A conventional rotating tire buffing rasp 200 is mounted on a verticalpedestal 202 situated on the backside of the machine as seen in FIG. 2.The rasp 200 is controlled via a conventional panel 204 to movelaterally along a desired buffing path 206 and horizontally towards andaway from the tire by conventional control mechanisms including a "joystick" used to control pneumatic cylinder 208, lead screws andassociated drive motors and the like. The buffer rasp 200 is rotated bya separate motor mounted on pedestal 202. The buffer mechanism, per se,is of a conventional type as marketed by Bandag, Inc., e.g. Buffer ModelNo. 23A.

An array of 16 ultrasonic acoustic receiving transducers 210 is disposedabove and around the outer walls of tire 112. The receivers 210preferably include a conically shaped collimator and/or focusing tube tohelp limit the field of view for each individual transducer to arelatively small and unique area across the tire wall. The receivers 210may be conveniently potted either individually or in groups in apolyurethane foam or the like to help mechanically fix the receivers intheir respective desired positions, to help protect the receivers and tohelp isolate the receivers from spurious ambient acoustic signals. Thearray of receivers 210 is radially adjusted into operative position byan air cylinder 212 having a coupled hydraulic control cylinder so as todefine a radially extended operative position for the receivers 210.

A block diagram of the combined tire buffer/NDI machine and itsassociated electrical and pneumatic circuits is shown in FIG. 3. Theelectrical motor and pneumatic cylinder controls 300 are of entirelyconventional design and thus not shown in detail. Operator inputsdepicted at the left of FIG. 3 are made directly or indirectly by theoperator via conventional electrical switches, relays, air valves and/orliquid control valves.

In operation, a tire is placed on lift 114 and raised into positionbetween the rings 108 and 110. Preferably, a predetermined indexposition on the tire is aligned with a physical index position on flange108. Thereafter, the chucking apparatus is engaged by causing flange 110to move into the tire 112 so as to pinch the tire beads together inpreparation for tire inflation. The tire is then inflated to a desiredset point pressure. The flange 108 is spring-loaded such that duringchuck engagement and tire inflation, it is caused to move axiallyoutwardly against the spring-loaded (e.g. by approximately 2 inches).This facilitates the tire inflation process and simultaneously uncoversan ultrasonic transmitter located within the tire from a relativelyprotected position so that it may subsequently be extended into anoperative position under the array of receivers 210. An interlock switchactivated by air pressure and/or by the physical movement of flange 108may be used to prevent any premature extension of the transmitter beforeit is uncovered from its protected position.

In the buffing mode, the transmitter need not be extended. The buffigrasp drive motors are conventionally activated and controlled (e.g. witha "joy stick" and conventional push button controls) to buff the tiretread surface as desired. Although it may not be required, it ispresently preferred to have the tire buffed to a substantially uniformouter treadwall surface before NDI operations are performed. Suchbuffing is believed to avoid possible spurious indications of defectscaused by normal tread patterns and/or by uneven wear about the tiresurface.

When the operator selects the NDI mode of operation, an ultrasonictransmitter located inside the inflated tire 112 is extended intooperative position and the array of receivers 210 is lowered intooperative position by respectively associated pneumatic cylinders. Thesame 2-horsepower d.c. motor which drives the tire at approximately 600surface feet per minute during buffing operations may be reduced inspeed by conventional electrical circuits so as to drive the tire atapproximately 40 surface feet per minute during the NDI mode. After thetire motion has reached a steady state, the operator may activate thescan request input switch to the ultrasonic NDI circuits 302. Thereafterthe walls of tire 112 will be ultrasonically inspected during one ormore complete tire revolutions to produce a display 304 which can behumanly interpreted directly or indirectly to reveal the condition ofthe tire (e.g., satisfactory for further buffing and retreading,doubtful or unsatisfactory). If questionable condition is indicated, thetire may be discarded or may be additionally buffed and retested.

The ultrasonic NDI circuits 302 are shown in greater detail at FIGS.4-10. As shown in FIG. 4, the outputs from the 16 ultrasonic receivers210 are amplified and multiplexed onto eight signal processing channelsA-H by circuits 402 which are shown in greater detail in FIG. 5. Eachsignal processing channel then provides AGC amplification,rectification, integration and analog-to-digital conversion with thesignal processing circuitry 404. A representative channel of suchprocessing circuitry is shown in detail at FIG. 7. The resultingdigitized outputs are presented to a conventional eight bit data bus 406which is interconnected to a conventional micro-computer CPU (e.g. an8080 type of eight bit computer) 408. The CPU 408 is also connected viaa conventional address bus 410 and data bus 406 to a data memory 412, toa programmable read-only memory (PROM) 414 and to a system interfacecircuit 416 which is shown in detail at FIG. 8. A display interface 418(shown in detail at FIG. 10) is directly connected to the data memorybanks 412 to provide a CRT type of oscilloscope display.

The system interface 416 provides the necessary gating and other controlsignals to the signal processing circuitry 404 and also provides HIGHCHAN multiplexing signals to the preamplifier circuits 402 as well as tothe transmitter drivers and multiplexing circuitry 422 used to driveplural ultrasonic transmitters. The operation of the entire system issynchronized to the rotational movements of tire 112 through a rotarypulse generator 424 directly driven with the tire (e.g. geared to thereducer gears). The rotary pulse generator 424 provides 1,024 pulses perrevolution at terminal RPGX and 1 pulse per revolution at terminalsRPGY.

As shown in FIG. 5, ultrasonic acoustic transmitting crystals 500 and502 are disposed inside inflated tire 112, which is chucked betweenrings 108 and 110, rotatably secured to spindles 102 and 104,respectively. The electrical leads feeding transmitters 500 and 502 arefed out through the fixed spindle 102 to the transmitter activationcircuits. Inflation air is likewise fed in through the center of spindle102 as are pneumatic lines and/or other control connections forextending and retracting the transmitters.

The exemplary ultrasonic transmitters 500 and 502 have a radiation fieldwhich substantially illuminates a sector of approximately 90°. Hence,they are mounted at 90° with respect to one another on block 504 whichmay, for example, be formed from polyvinyl chloride plastic materials.It has been found that acceptable operation will not result if thetransmitters are too close to the inside tire surfaces or too far awayfrom these surfaces. In the preferred exemplary embodiment, transmittingcrystals 500 and 502 are approximately two inches from the inner tirewall surfaces although this optimum distance of separation may be variedby a considerable amount (e.g. plus or minus approximately one inch).

The arrayed receiving transducers 210 are located about an arc generallycorresponding to the outside shape of the tire wall. Here again, it hasbeen found that acceptable operation does not result if the receiversare too close or too far away from the outer tire walls. Preferably, thereceivers are no closer than approximately 1 inch to the outer tiresurface but are preferably within 5.5 to 8.5 inches of the opposinglysituated transmitting crystal. The receiving transducers 210 preferablyeach employ a conically shaped collimator and/or focusing tube as shownin detail at FIG. 12. These tubes are preferably machined from polyvinylchloride plastic material and also help to match the impedance of theactual transducer crystal surface to the surrounding ambient airacoustic impedance.

A moderately high ultrasonic frequency is employed so as to help avoidinterference from spurious ambient acoustic signals and to obtainincreased resolution by using shorter wavelength acoustic signals whileat the same time avoiding ultra-high frequency acoustic signals and theproblems associated therewith. Frequencies above 40 kHz are desirablewith 75 kHz being chosen as the presently preferred optimum frequency.Ultrasonic transducing crystals operating at 75 kHz are conventionallyavailable. For example, receiving crystals are available as the MK-111transducer from Massa Corporation, Windom, Mass., having the followingspecifications:

    ______________________________________                                        Frequency of Maximum                                                          Impedance (fm)       75 kHz ± 3 kHz                                        Impedance at fm (min 6K Ohms                                                  Receiving Sensitivity (O.C.)                                                  at Frequency of Max Output                                                    Db re 1 Volt/microbar                                                                              -70DB min.                                               Transmitting Sensitivity                                                      Db re 1 microbar at 1                                                         ft./10mw             -12DB Min.                                               Maximum Power Input  100MW                                                    Directivity          -10DB Max. at                                                                 90° Total Angle                                   Temperature Stability                                                                              10% Change in                                                                 Frequency -30° F.                                                      to +150° F.                                       Capacitance          1200 ± 20% PF                                         ______________________________________                                    

A suitable transmitting crystal tuned to approximately 75 kHz isavailable from Ametek/Straza, California under No. 8-6A016853.

The electrical leads from each of the transducers 210 are preferablyconnected through coaxial cables 506 to their respectively associatedpre-amplifiers 508. The outputs from each of the 16 amplifiers 508 areconnected to an eight pole double throw electronic switch comprisingSignetics SD5000 integrated circuits, controlled by the HIGH CHANmultiplexing signal provided by system interface 416. The eightresulting multiplexed output channels are connected through transistorbuffer amplifiers to signal processing channels A-H. Accordingly, in theabsence of a HIGH CHAN multiplex signal, the outputs from the firsteight preamplifiers 508 are coupled to respectively corresponding signalprocessing channels A-H. However, when the HIGH CHAN multiplexing signalis present, the outputs from the last eight of the pre-amplifiers 508are connected to respectively corresponding signal processing channelsA-H.

The circuitry of each pre-amplifier 508 is shown in more detail at FIG.6. It includes a first transistorized stage having a gain ofapproximately 150 followed by a cascaded integrated circuit amplifierhaving a gain factor of approximately 11.

The signal processing circuits 404 for each of channels A-H areidentical. Accordingly, only the circuitry for channel A is shown inFIG. 7. The waveforms shown in FIG. 11 will be useful in understandingthe operation of the circuitry in FIG. 7.

The generation of a pulsed or bursted ultrasonic waveform for drivingthe transmitters 500 and 502 will be described later. However, byreference to FIG. 11, it may be seen that each transmitter is driven toprovide at least one approximately 50 cycle burst of 75 kHz acousticoutput signals each time an RPGX trigger pulse occurs (e.g. 1,024 timesper tire revolution). After a transmission delay, which will depend uponthe separation between transmitter and receiver and the characteristicsof the intervening ambient air and tire rubber, the transmitted acousticsignals are received. The received and transduced acoustic signals mayhave a complex amplitude envelope (rather than the well-behaved oneshown in FIG. 11) depending upon the type of multiple reflections,internal reverberations, wave cancellations, and/or other peculiar waveeffects which take place along the transmission path. Accordingly, it isonly the leading edge or initial portion of each such ultrasonic pulseor burst (e.g. where the amplitude envelope is initially increasing)that provides the best and most accurate indication of the transmissionpath quality (i.e. its included tire structural defects). Accordingly,the signal processing circuitry shown in FIG. 7 is adapted toeffectively utilize only such initial or leading edge portions of eachburst of ultrasonic signals. In one embodiment, data for each tiremeasurement area is obtained by averaging measurements taken atdifferent respective acoustic frequencies.

As explained in U.S. Pat. No. 3,882,717, it is necessary to provideautomatic gain control amplification of through-transmission ultrasonictest signals to compensate for different average tire casingthicknesses. This earlier patented system had but a single signalprocessing channel with AGC employed to compensate for differences inaverage tire casing thicknesses over the cross-section of a given tire.However, it has been discovered that automatic gain controlledamplification must also be included in each of the plural testingchannels of this invention so as to compensate for differences inaverage tire casing thickness from tire-to-tire.

Accordingly, an AGC amplifier 700 (e.g. integrated circuit MC1352) isincluded within channel A as shown in FIG. 7. The ultrasonic signalspassing through channel A are fed back to pin 10 of the AGC amplifier700 and input to a relatively long time constant (e.g. 10 seconds) RCcircuit 702 connected to pin 9 of amplifier 700. Accordingly, theaverage of signals passing through the channel over the last severalseconds (during the included periods that the amplifier is enabled) iscompared to a constant reference AGC bias presented at pin 6 so as tomaintain a substantially constant average output level at pin 7 over theRC time constant period. Amplifier 700 in the preferred exemplaryembodiment has a gain which may vary automatically between a factor of 1and 1000.

Amplifiers 704 and 706 are connected in cascade within channel A andeach provide a gain factor of approximately 2. Additionally, amplifier706 has diodes 708 and 710 connected so as to effect a full waverectification of its output signals as presented to the FET gate 712.

Referring back to FIG. 11, an integrate reset signal INTGRST isgenerated during the first transmission delay period for a given testtire position and presented to FET gate 714 (FIG. 7) so as to dischargethe integration capacitor 716 connected across amplifier 718 (forming aMiller-type integrator). Furthermore, the AGC amplifier 700 is enabledby the AGCEN signal at some point during each testing cycle so as tosample the received signals. The integrator enabling signal INTGEN istimed so as to enable the FET switch 712 only during the initialportions or leading edge of the ultrasonic burst (e.g. approximately 130microseconds or about the first 10 cycles of the 75 kHz burst). Ifdesired, two or more received bursts at respective different frequenciesmay be sampled and the results integrated together so as to effectivelyaverage measurements taken at different frequencies (and hence havingdifferent acoustic standing wave patterns).

Thereafter, the output of integrator 718 is converted to a digitalsignal under program control by CPU 408 generating suitable analog DACinputs to comparator 720 and conversion gating signals CONV to gate 722which interfaces with one of the conventional data bus lines (in thiscase DBφ. Such program controlled analog-to-digital conversion isconventional and involves the CPU program controlled conversion ofreference digital signals to reference analog DAC signals which are thensuccessively compared in comparator 720 with the results of suchcomparisons being made available to the CPU via data bus lines and gates722. By a process of successive comparisons to different known referencesignals, the programmed CPU is capable of determining a digital valuecorresponding to the input integrated analog value from amplifier 718.

This process is of course repeated simultaneously in channels A-H andsuccessively in each channel for each burst or group bursts ofultrasonic signals occurring at a given tire wall test site.

Referring now to FIG. 8, the RPGX (1,024 pulses per revolution) and RPGY(1 pulse per revolution) signals from the rotary pulse generator arepassed through tri-state buffers 800 to data bus lines DB0 and DB1respectively in response to the IN3 and Q4 addressing signals providedby the CPU. Other addressing outputs from the CPU are input to an outputdecoder 802 so as to provide signals OUT320.00 through OUT320.70 underappropriate program control.

Just prior to a scan cycle, the CPU is programmed to repetitively polldata bus line DB2 looking for a scan request signal SCANRQ generated byan operator manipulation of the scan request switch 804 which causesflip-flop 806 to be set at the next occurrence of OUT320.60.

Once a scan request has been detected by the CPU via data bus line DB2,the CPU is programmed to poll the RPGX and RPGY signals which are thenpresented on data bus lines DBφ and DB1 by address inputs IN3 and Q4. Anactual measurement cycle is not started until the second RPGY signal isdetected so as to insure that the tire is running true at asubstantially steady state speed and that the AGC circuits are operatingproperly. Thereafter, each occurrence of an RPGX signal detected by theCPU is programmed to cause the generation of an OUT320.10 signal. TheOUT320.10 signal triggers one shot circuits 808 and 810 and also enablesthe latch 812 to accept the digital values presented on data bus linesDBφ through DB4.

Just prior to the generation of the first burst of ultrasonic waves at agiven tire wall test site, the CPU generates OUT320.70 which triggersreset one shot 822 and provides an integrator reset signal INTGRST viaaddressable flip-flop 823 and NAND gate 825.

The 4 bit binary counters 814 and 816 are connected in cascade to countthe 18.432 Mhz clock signals input from the CPU board and to dividethese clock pulses by a numerical factor represented by the contents oflatch 812. The result is an approximately 75 kHz clock signal, (both 74kHz and 76 kHz frequencies are used successively in one embodiment withthe two results averaged together) which is used to trigger one shot 818having an adjustable time period such that its output can be adjusted toa substantially square wave 50% duty cycle signal. As shown in FIG. 8,one shot 818 is controlled by a pulser enabling signal from theaddressable flip-flop 819. Thus if desired (e.g. to listen for leaks),the ultrasonic transmitters may be selectively disabled by the CPU.

The approximately 75 kHz 50% duty cycle signal is then buffered throughamplifier 820 and presented as square wave output MB (see FIG. 11) toconventional transmitter driver amplifiers (providing approximately 200volts peak-to-peak electrical output) which, in turn, cause a generallysinusoid type of 75 kHz acoustic output from the transmitter as shown inFIG. 11.

This generation of the approximately 75 kHz output MB will continueuntil one shot 808 times out (e.g. approximately 1 millisecond). Duringthat interval, a burst of ultrasonic acoustic signals is caused toemanate from one of the transmitting crystals.

The period of one shot 810 is adjusted for a delay approximately equalto but slightly less than the transmission delay between acoustictransducers. The delayed output from one shot 810 resets the data readyflip-flop 828 and triggers the integrate timing one shot 826 whichproduces the integrate enable signal INTGEN. At the conclusion of theintegrate enable signal from one shot 826, the data ready flip-flop 828is set to provide a data ready signal to the CPU via data bus line DB4.If more than one analog data value is to be combined at the output ofthe integrator, the CPU is simply programmed to ignore the data readysignal until the requisite number of measurement cycles have beencompleted. Ultimately, however, the data ready signal indicates to theCPU that analog-to-digital conversion of the integrated analog signal isnow ready to be performed. The CPU, under conventional program control,then begins to produce various analog reference signals DAC from thedigital-to-analog converter 830 under control of the digital datalatched into latch 832 from the data bus lines by the addressing signalOUT320.00. At the same time, the CPU is programmed to provide properconversion gating signals CONV via the addressing inputs to gates 834,836 and 838.

The DAC may be a linear type 08 or a non-linear exponential type 76 orother known non-linear types of DAC circuits. The non-linear DAC-76 isbelieved to improve the effective signal-to-noise ratio for lower levelsignals.

The CPU is programmed so as to normally produce the multiplexing HIGHCHAN output by setting and resetting the addressable flip-flop 840 viathe address lines Aφ-A2, OUT320.30 in accordance with the data valuethen present on data line DBφ. However, manual override switch 842 hasbeen provided so that either the low channels φ-7 or high channels 8-15may be manually forced via tri-state buffers 844 with outputs connectedto the data bus lines DB6 and DB7.

The flow diagram for an exemplary CPU control program is shown in FIGS.15-16. Conventional power-up, resetting and initialization steps areshown at block 1500. After the START entry point, the scan requestflip-flop 806 (FIG. 8) is reset, the integrators are disabled (viaflip-flop 823, FIG. 8), and the data memory circuits are disabled atblock 1502. Thereafter polling loop 1504 is entered and maintained untila SCANRQ on DB2 is detected.

Once a scan request has been detected, the indicator lamps are tested,the integrators are enabled for normal operation (via flip-flop 823),the data memory is enabled for access by the CPU (and conversely, thedisplay interface is disabled from access to the data memory) at block1506. The high/low/normal switch 842 (FIG. 8) is also checked via DB6and DB7. If the low or normal mode is indicated, the HIGH CHAN multiplexsignal is maintained equal to zero via flip-flop 840. Thereafter,polling loop 1508 is entered to test for an RPGY transition. A similarpolling loop 1510 is subsequently entered to issue at least one tirerevolution before measurements are taken. Then a software counterθ_(current) is set to zero and the LOOP1 testing subroutine (FIG. 16) isentered. As will now be explained in more detail, the step within LOOP1are executed 1024 times to collect and record 1024 data values in eachof eight transducer channels corresponding to 1024 tire testing sitesdistributed over a whole 360° of tire rotation in each of the eightchannels.

After entry of LOOP1, the RPGX signal on DBφ is tested for a transitionfrom 1 to φ at loop 1600. Once this transition occurs, all theintegrators are reset (via one shot 822, FIG. 8), the latch 812 is setto produce a 74 kHz MB drive signal and the transducers are driven witha burst of 74 kHz MB drive signals via one shot 808 and a pulserenabling signal via flip-flop 819. Since one shot 810 is also triggered,the leading edge of the received burst is gated and integrated in eachchannel.

While this test at 74 kHz is being performed, the CPU is in a wait loop1602. Thereafter, latch 812 is reset to produce a 76 kHz MB signal andthe transmitters are again pulsed. The result is another gatedintegration of the leading edge of a received burst at 76 kHz. As soonas this second integration is completed, the data ready signal on DB4 isdetected at waiting loop 1604. After the analog data has thus beenaccumulated for two different frequencies at a given tire test site, theAGC circuits are keyed (to keep them actively sampling the channelsignal level within the relevant RC time constant period) and aconventional analog-to-digital conversion routine is entered. Thisroutine converts each integrator output to a six bit digital value whichis then stored in the data memory 412. The data for each channel isstored in a separate section of the memory so that similar data pointsfof each channel can be later addressed using the same lower ordermemory addressing signals.

The θ_(current) software counter is thereafter incremented by one andLOOP1 is re-entered unless data measurements at all 1024 tire test siteshave already been taken.

After the first exit from LOOP1, a pattern recognition subroutine may beentered, if desired, at block 1512. The pattern recognition results maythen be tested at 1514 and 1516 to determine which of status indicatorlamps 846 (FIG. 8) should be lighted. Alternatively, the patternrecognition steps may be skipped as shown by dotted line 1518 to flipthe HIGH CHAN multiplex signal, if operation is in the normal mode. (Ifonly high or low channel testing has been forced by switch 842, returncan now be made to the START entry point.) Thereafter, measurements aretaken for the higher group of eight channels as should now be apparent.

While LOOP1 in FIG. 16 causes measurements at 74 KHz and 76 KHz to becombined, it should also be apparent that block 1606 can be skipped ifmeasurements at only a single frequency are desired. Similarly,measurements at more than two frequencies can be combined if desired.Furthermore, the combination of plural data values can be initially madeeither in analog form (as in the exemplary embodiment) or in digitalform as should now be apparent.

As already discussed, the cpu may be programmed, if desired, toautomatically analyze the digitized data collected during a completescanning cycle with pattern recognition algorithms and to activate oneof the indicator lamps 846 (e.g., representing acceptance, rejection orair leakage) via conventional lamp driving circuits 848 as controlled bythe contents of latch 850 which is filled from data bus lines DBφthrough DB4 under control of the address generated OUT320.20 signal. Airleakage can be detected, for example, by performing a complete scanningand measurement cycle while disabling the ultrasonic transmitters.Detected increases in received signals are then detected as leaks.

The central processing unit shown in FIG. 9 is conventionally connectedto decode the various address lines and provide addressing inputsalready discussed with respect to the system interface shown in FIG. 8.The CPU itself is a conventional integrated circuit 8080 microprocessorhaving data input and output lines Dφ through D7 which are connected tothe data bus lines DBφ through DB7 through conventional bi-directionalbus driver circuits 900. Address lines Aφ through A9 and A13 are alsodirectly connected through buffer amplifiers 902 to the systeminterface, memory circuits, etc. Address lines A10, A11 and A12 aredecoded in decoder 904 to provide addressing outputs Qφ through Q7.Similarly, addressing lines A14 and A15 are decoded together with thenormal writing and data bus input signals from the CPU in decodercircuitry 906 to provide INφ through IN3 and OUTφ through OUT3addressing outputs. The normal data bus input CPU signal DBIN and theaddressing lines 814 and 815 are also connected through gates 908 and910 to conventionally provide a directional enabling input to thebi-directional bus drivers 900. The approximately 18 Mhz clock 912 isalso conventionally connected to the 8080 CPU. However, pin 12 of the3G8224 integrated circuit is brought out to deliver an 18.432 Mhz clockto the frequency dividing circuits of the system interface alreadydiscussed with respect to FIG. 8.

The data memory circuits are provided by a conventional connection of 25integrated circuits of the 4045 type so as to provide 8,192 eight bitbytes or words of data storage capability.

The programmable read-only memories may be provided by three integratedcircuits of the 2708 type, each providing 1,024 bytes of programmedmemory. 256 eight bit words of read/write memory are also preferablyconnected to the CPU as part of the programmable memory circuits. Anintegrated circuit of the type 2111-1 may be used for this purpose.

The CRT display interface is directly connected to the data memoryboard. Once an entire measure cycle has been completed (e.g. when thethird RPGY signal has been detected after a scan request), there are1,024 data values available for each of the 16 measurement channelsrepresenting the relative magnitudes of ultrasonic signals transmittedthrough the tire at 1,024 successive respectively correspondingpositions about the tire circumference within the area monitored by thereceiving transducer for a given channel. This digital data may beconverted to conventional video driving signals for a CRT and displayedas shown in FIGS. 13 and 14. Alternatively, the 8080 computer may beprogrammed to analyze (e.g. by pattern recognition algorithms) theavailable digital data and to activate appropriate ones of the indicatorlamps 846 shown in FIG. 8.

The display interface shown in FIG. 10 is conventionally connecteddirectly to the data memory 412 via memory data bus lines 1000, memoryquadrant selection bus lines 1002, memory address bus lines 1004 anddata latch strobe line 1006. The whole display can be selectivelydisabled or enabled as desired under CPU control via CPU addressingoutputs A13, Q3, OUT3 and Aφ via flip-flop 1008 and the associatedinverter and gates shown in FIG. 10. In the preferred embodiment, thedisplay interface is disabled whenever other parts of the system areaccessing the data memory 412 so as to prevent possible simultaneousactivation of the data memory circuits.

The display interface is driven by a 11.445 MHz clock 1010. Its outputdrives counter 1012 which is connected to divide the clock signals by afactor of 70. The first 64 counts of counter 1012 are used by comparator1014 which also receives 6 bits of data (i.e. 64 different numericalvalues) from the addressed data memory location representing themagnitude of ultrasonic signals transmitted through a particular tiretesting site. Thus the output from comparator 1014 on line 1016 willoccur at a specific time within 64 clock periods corresponding to themagnitude of the input digital data via lines 1000. The clock pulseduring data coincidence will cause flip-flop 1018 to transitionmomentarily and produce a video output pulse via gate 1020 having onedisplay dot time width and spaced within its respectively correspondingchannel time slot according to the magnitude of the recorded data.Flip-flop 1022 is triggered by counter 1012 upon counting a 65th clockpulse and generates an inter-channel separation blanking video pulse outof gate 1020. The counter 1012 then continues to count 5 more clockpulses before resetting itself and starting another cycle using datafrom the next adjacent channel.

The 70th count from counter 1012 also drives a three bit channel counter1024 which, through the 3-to-8 decoder 1026, successively addresseseight different sections of the data memory corresponding respectivelyto eight of the sixteen ultrasonic receiver channels. A selectionbetween display of the higher or lower eight channels is made via switch1028.

At the end of a complete horizontal scan line, 8×70 clock pulses (2×70clock pulses are counted during horizontal retrace period) will havebeen counted by counters 1012 and 1024 and a carry pulse will go to the12 bit counter 1029 so as to increment the addresses on line 1004 (viadecoder 1030) for the next horizontal scan line. In the case of theusual interlaced CRT scanning raster, every other horizontal line willactually be skipped and picked up during a second vertical seam rasteras will be appreciated. The states of counters 1024 and 1029 provide allrequisite timing information for conventionally generating the usual CRThorizontal synchronization, vertical synchronization and vertical andhorizontal retrace blanking video signals at 1032.

The various video signals are conventionally mixed in video amplifier1034 and output to a CRT display.

Since there are 1024 data values in each channel but many fewerhorizontal scan lines iln the usual CRT raster, switch 1036 is providedso as to select only the odd or even addresses for data values in agiven channel. Thus the complete 360° of scanned tire surface, within agiven channel, is displayed in an assigned time slot over 512vertically-spaced horizontal scan lines.

As thus described, the data values for a given channel would bedistributed within a vertical segment of the CRT display and displacedin a horizontal sense from a vertical base datum line in accordance withthe stored data values. However, in the preferred embodiment, the CRTdeflection yoke is rotated by 90° so that the final CRT display for achannel is presented horizontally as shown in FIGS. 13 and 14.

As depicted in FIGS. 13 and 14, the signal traces in each individualchannel are deflected upwardly to represent reduced ultrasonic signalmagnitudes. Accordingly, in FIG. 13, it can be seen that a defect hasoccurred in channels 12 and 13 a at approximately 20° from the indexmarker. Similarly, a defect is shown in FIG. 14 at channels 12, 13 and14 at approximately 280°.

Although not shown in FIGS. 13 and 14, if a leak had been present, itwould have been indicated by an increased signal magnitude which, in therepresentation of FIGS. 13 and 14, would have resulted in a downwarddeflection of the signal trace for the corresponding channel.

The tracing for channels φ through 3 and 12-15 is caused by wire ends,transitions between various normal tire layers and a periodic pattern ofremaining tire tread structures about the outer edges of the tiretreadwall. The data actually shown in FIGS. 13 and 14 was taken using alinear DAC circuit in the analog-to-digital conversional process.

FIG. 17 shows another circuit for generating the AGC amplifier andintegrator channels. The circuit permits generation of INTGEN, AGCEN,INTGRST, and MBT pulses from RPGX pulses at 1605 or simulated RPG pulsesfrom addressable latch 1608 under program control.

When the RPG simulator is enabled, 1608 output labeled 5 is a 50% dutycycle pulse train which is selected by multiplexor 1611 to triggerone-shots 1612 and 1613. One-shot 1612 is triggered by the rising edgeof the output of 1611 and times out in 300 ns. One-shot 1613 istriggered by the falling edge of 1611 and also times out in 300 ns.

The outputs of 1612 and 1613 are combined to trigger DELAY one-shot 1614and MB one-shot 1615. The generation of 75 KHz bursts by 1615, 1620,1621, 1622 and 1623 has been previously described. DELAY one-shot 1614triggers INTEGRATE one-shot 1616 and resets DATA READY flip flop 1617.

Flip flop 1617 signals that the analog outputs of the AGCamplifier/integrator channels are ready for digitizing. Flip flop 1617is only set while RPG is high.

Flip flop 1617 triggers AGCEN flip flop 1619 which is level shifted andsent to the AGC amplifiers.

A delayed RPG signal appears at the output of flip flop 1618 and it isused by the software for synchronizing to tire rotation.

When the simulator is disabled, multiplexor 1611 sends the logicaloutput of 1605 to one-shots 1612 and 1613. The input source for 1611 nowcomes from the tire-rotation generated RPGX pulses, and the generationof the required outputs, i.e., INTGEN, is accomplished by controllingmultiplexor 1611 outputting pulses to one-shots 1612 and 1613.

The sequence of one-shot firings follows the same pattern as describedin the previous paragraphs when the RPG simulator is activated.

The DAC comprised of 1624 and 1625 generate an analog voltage used bythe CPU for analog-to-digital conversion of the integrated values ofreceived signals.

Decoder 1609, flip flop 1610, register 1627 and lamp driver 1628 performfunctions already described. Latch-decoder decoder 1629 and display 1630provide status information during program execution.

During air-leak detection, PULSEN generated by software at 1608 is low,thus inhibiting MB excitation pulses to the pulser unit by clearingone-shot 1620.

FIGS. 18, 19a and 19b illustrate a program sequence which searches forair leaks then searches for separations in two eight-channel groups.

Blocks 1631 and 1632 initialize states of the system and 1633 selectsthe RPG simulator to trigger the one-shot timing elements. The RPGsimulator switches alternately high and low at a 8 ms rate while theSCAN RQ flip flop is tested in the loop 1634 and 1635. The RPG simulatorrefreshes the AGC levels so when SCAN RQ becomes active, dataacquisition for air leaks can begin immediately.

When SCAN RQ becomes active, the RPG unit is selected in 1636 and datamemory enabled in 1637. Subroutine GETDATA is called at 1638 and isdetailed in FIGS. 20a and 20b. Next, PATTERN REC is called at 1639, andany air leaks present will be detected and the AIR LEAK lamp will beturned on by 1640 and 1641.

Now the pulser is activated at 1642. Tests for HICHAN, LOCHAN only andnormal scan are done at 1643, 1644 and 1645.

Subroutines GETDATA and PATTERN REC are called at 1646. Blocks 1647,1648, 1649 and 1650 test for REJECT/ACCEPT status and decide whether tocontinue to scan the high channel group. GETDATA and PATTERN REC arecalled again at 1651 and the tire status is tested again by 1652 and theprogram returns to CONTINUE via 1653, REJECT status, or 1654, ACCEPTstatus.

FIGS. 20a and 20b detail the flow of subroutine GETDATA. The positioncounter, θ CURRENT is set to zero at 1655. The tire scan begins at thecurrent tire position which is assumed to be the origin. Block 1656tests for occurrence of the once-per-revolution INDEX pulse and stores θCURRENT at location OFFSET. If INDEX is present, then 1657 stores thelocation in memory.

Block 1658 waits until RPG is zero. When the condition is met, 1659 setsthe pulsing frequency to 74 KHz and repeats the INDEX test at 1660 and1661, and waits until RPG is one at 1662. A new pulsing frequency isselected at 1663.

When a complete RPG cycle has elapsed, the DATA READY flip flop will beset, and 1664 waits for this condition. When DATA READY is true, eightsteady state voltages generated by each of the integrators are convertedby block 1665 and stored in data memory as raw data. The tire positionis incremented and tested for the last data point at 1666. The programcontinues to acquire data by jumping to the reentry point B. When allpoints are digitized and stored, the data is justified in memory by 1667so the data associated to the INDEX point is at the start of the datablock.

While only a few exemplary embodiments and only a few variations thereofhave been explained in detail, those in the art will appreciate thatmany other modifications and variations may be made without departingfrom the novel and advantageous features of this invention. Accordingly,all such modifications and variations are intended to be included withinthe scope of this invention as defined by the appended claims.

What is claimed is:
 1. Tire testing apparatus for detecting air leaksfrom pressurized tires, said apparatus comprising:an ultrasonictransducer for converting ultrasonic vibrations in excess of about 40KHz into corresponding electrical signals; means for mounting and movingthe wall of an inflated tire past said ultrasonic transducer; and signalprocessing means connected to said transducer for detecting the presenceof ultrasonic signals in excess of about 40 KHz generated by airescaping from a leak in the tire wall.
 2. Tire testing apparatus as inclaim 1 wherein said ultrasonic transducer and signal processing meansdetect ultrasonic signals of approximately 75 KHz in frequency.
 3. Tiretesting apparatus for detecting leaks from a pressurized tire wall, saidapparatus comprising:an array of plural ultrasonic transducers mountedto simultaneously scan in proximity to corresponding parallel elongatedsegments of at least a portion of said pressurized tire wall; andelectrical signal processing circuitry connected to said ultrasonictransducers for detecting respectively associated ultrasonic signals ofapproximately 75 KHz generated by leaks from corresponding segments ofsaid portion of said pressurized tire wall.
 4. Tire testing apparatuscomprising:tire mounting means for inflating with air and rotatablyholding a tire thereon; at least one ultrasonic transducer mounted toscan said inflated tire as it is rotated therepast; and electricalsignal processing means for detecting only electrical outputs having afrequency of about 75 KHz from said ultrasonic transducer caused by airescaping from a leak in said tire as it passes in proximity to saidtransducer.
 5. Tire testing method for detecting air leaks frompressurized tires, said method comprising:mounting and moving the wallof an inflated tire past a stationary ultrasonic transducer whichconverts ultrasonic vibrations in excess of about 40 KHz intocorresponding electrical signals; and detecting the presence ofultrasonic signals generated by air escaping from a leak in the tirewall only if in excess of about 40 KHz.
 6. Tire testing method as inclaim 5 wherein said detected ultrasonic signals are approximately 75KHz in frequency.
 7. Tire testing method for detecting leaks from apressurized tire wall, said method comprising:simultaneously scanning inproximity to parallel adjoining segments along at least a portion ofsaid pressurized tire wall with an array of plural ultrasonictransducers; and detecting ultrasonic signals of about 75 KHz generatedby leaks from said portion of said pressurized tire wall.
 8. Tiretesting method comprising:inflating with air and rotatably scanning saidinflated tire with an ultrasonic transducer as it is rotated therepast;detecting electrical outputs from said ultrasonic transducer in excessof about 40 KHz caused by air escaping from a leak in said tire as itpasses in proximity to said transducer.
 9. Tire testing method as inclaim 8 wherein said detected ultrasonic electrical signals are offrequency of about 75 KHz.